Conventional force sensing transducers generally accommodate the applied force by means separate from those associated with a generation of the related electrical signal output. A variety of known transducers have been employed to detect and measure a variety of forces, each transducer producing an output signal roughly proportioned to the force measured. Many of the known transducers encounter problems of nonlinearity within the range of expected measurement values because what they measure is not in fact the direct effect of the force upon the output generating means. In most cases the force must first act on some other component or system within the transducer, or related to the environment in which the transducer, is placed, before there is any effect at all on the electrical signal generating means.
Some force sensing transducers employ a prestressed vibrating wire type strain gauge to create a frequency based output proportional to the stress in the vibrating wire. Prestressed wire type transducers exhibit one significant advantage over other types of force sensing transducers in that their electrical output signal format is frequency rather than analog magnitude related, and as such is largely insensitive to cable leakage, resistivity changes, etc. As a result of this attribute the reading of such instruments over several kilometers of cable is possible without downgrading of accuracy. For this reason the vibrating wire type of device is favored in the hostile environment of civil engineering applications particularly with regard to foundations and earthworks. However the prestressed vibrating wire device discussed above inherently suffers certain limitations which over the life of the installed, and often irrecoverable, transducer can and has raised doubts in users regarding the long-term stability of such units. One of the potential problems relates to long-term creep or loss of tension in the wire, whereby with time the stress level in this prestressed component decreases independent of applied force, and thus lowers the wire's resonant frequency.
Since this type of transducer, as discussed above, senses applied force as a relief of tension in the prestressed wire which reduces the resonant vibrating frequency of the wire, and since reduced frequency is usually indicative of increased applied force, such a long term decrease in wire stress, and hence resonant frequency is taken by the user to be an increase in applied force, whereas in reality all readings subsequent to tension loss are in error by the unknown extent of this tension loss.
The dimensions of this potential limitation can be illustrated by considering a typical vibrating wire pressure transducer with the following characteristics:
Range =0-100 psi
Diameter of wire =0.009 inch
Length of wire =1.5 inch
Frequency output range =4000.times.f.sup.2 /1000 digits
where f.sup.2 /1000 refers to a preferred readout mode wherein f.sup.2 is the square of the vibrational frequency and f.sup.2 /1000 is a single unit for readout purposes. In such a case, wire prestressing during assembly will stretch it by about 3 thousandths of an inch to achieve the zero applied pressure frequency of about 3000 Hz (9000 f.sup.2 /1000 indicated reading).
As the unit is externally loaded to full range pressure (100 psi) the pretensioned wire stress will reduce as the diaphragm strains to the applied pressure. This equates to approximately 1.2 thousandths of an inch strain over the free wire length, and as such represents the response over the full operating range of the transducer.
In terms of strain therefore, only one hundredth part 0.0012 inch, or 0.000012 inch component creep, in the diaphragm, body, wire or wire gripping points is necessary to produce an offset in frequency equivalent to 1% transducer zero drift. In terms of wire stress, only 0.4% of loss in wire prestress is necessary to cause a 1% transducer zero drift. In most civil engineering applications this cannot be detected or quantified after the unit is irretrievably installed for use.
As with most established types of force sensing transducers employing strain measuring elements separate from force absorbing components, the prestressed vibrating wire device design discussed above requires special attention to be paid to the relatively different temperature coefficient factors of the various components, in order to minimize performance changes which occur where the temperature effects on the various components do not match the effects on the prestressed wire.
Where temperature effects do not match, differential expansion or contraction occurs and causes output frequency changes to occur which cannot be differentiated from real applied force variations. This is so even in cases where there are careful design efforts to select the correct proportion of different materials to match the temperature response over a certain range of temperatures, because all components are not necessarily at the same temperature at any given time.
Temperature gradients across the various components, as the influencing temperature changes, will cause significant though transient errors in the magnitude of applied force indicated. This factor is usually of greatest significance at the time of installation where the instrument is often subjected to changes of temperature environment as placement occurs, and on such occasions the registering of erroneous datum readings before temperature stabilization occurs is not uncommon.
Other known force sensing transducers employ a relatively unstressed vibrating wire instead of a prestressed wire where the force to be measured is applied directly to the wire. However even though some of the difficulties discussed above are avoided by employing a wire which is not prestressed, devices employing hard musical wire still experience wire creep and crimp slippage even under relatively low stress levels. That is because hard steel wire must be crimped or swaged under extremely high stress levels in order to ensure gripping of the wire at the highest anticipated stress level. Under these extremely high stress levels stress is inevitably transferred longitudinally into the wire causing unavoidable wire deformations with the resulting problems as discussed above. Moreover the high stress levels force the crimp itself into plasticity, with the eventual result over time of wire slippage in the crimp resulting in the same kinds of errors discussed above for prestressed wires.
Accordingly it is an object of the invention to measure the magnitude of a force by applying it directly and as completely as possible to a vibrating strip which bears virtually all of the force to mechanically provide means by which a frequency related, rather than analog based, electrical output signal is derived directly from the stress in the strip.
It is another object of the invention to accommodate an applied force and generation of a related electrical signal output by one in the same means.
It is a further object of the invention to significantly minimize the effect of long-term creep or tension loss discussed above by employing material such as a thin steel strip which is not prestressed.
It is a still further object of the invention to provide a vibrating strip which is inherently less sensitive to thermal expansion and to temperature gradient effects than the transducers discussed above.
It is another object of the invention to provide increased reading resolution for full rated pressure range.
It is a further object of the invention to provide an invention with a design which results in uniformity of design for a variety of applied force ranges and in low completed item wastage rates.
These and other objects of the invention are accomplished according to the disclosure of the invention herein described as more fully set forth below.